The bodies were found by police in the Zaragoza municipality, located in the northwestern Antioquia department.
According to locals in the area, the four victims had arrived from the southwestern Valle del Cauca department, the heartland of the Rastrojos, just days before the shooting.
The bodies would be transferred to the morgue of Zaragoza for further analysis, reported radio station Caracol.
Fighting between the two groups has been reported in several Colombian departments. In early January, combat between them displaced some 800 people in the western Choco department. The drug traffickers are also reportedly locked into a deadly struggle to control the strategic Valle del Cauca department.